The present invention relates to a storage system configured to copy data to a remote location.
Data is the underlying resources on which all computing processes are based. With the recent explosive growth of the Internet and e-business, the demand on data storage systems has increased tremendously. Generally, storage networking encompasses two applications or configurations: network-attached storage (NAS) or storage area network (SAN). A NAS uses IP over Ethernet to transports data in file formats between storage servers and their clients. In NAS, an integrated storage system, such as a disk array or tape device, connects directly to a messaging network through a local area network (LAN) interface, such as Ethernet, using messaging communications protocols like TCP/IP. The storage system functions as a server in a client-server system.
Generally, a SAN is a dedicated high performance network to move data between heterogeneous servers and storage resources. Unlike NAS, a separate dedicated network is provided to avoid any traffic conflicts between client and servers on the traditional messaging network. A SAN permits establishment of direct connections between storage resources and processors or servers. A SAN can be shared between servers or dedicated to a particular server. It can be concentrated in a single locality or extended over geographical distances. SAN interfaces can be various different protocols, such as Fibre Channel (FC), Enterprise Systems Connection (ESCON), Small Computer Systems Interface (SCSI), Serial Storage Architecture (SSA), High Performance Parallel Interface (HIPPI), or other protocols as they emerge in the future. For example, the Internet Engineering Task Force (IETF) is developing a new protocol or standard iSCSI that would enable block storage over TCP/IP, while some companies are working to offload the iSCSI-TCP/IP protocol stack from the host processor to make iSCSI a dominant standard for SANs.
Regardless of the types of storage system used, the data storage system users are interested in maintaining and/or copying data to a secondary site at a remote location. One reason for this is to provide a back-up data in order to prevent loss of valuable data from failure in a primary storage unit (or storage subsystem). Another reason is to provide a decision support process. Yet another reason is to perform data migration.
Currently, two operational modes are used by storage systems to copy the data to the secondary sites: synchronous mode and asynchronous mode. In synchronous mode, a write request from a host to the primary storage system completes only after write data are copied to the secondary storage system and acknowledge thereof has been made. Accordingly, this mode guarantees no loss of data at the secondary system since the write data from the host is stored in the cache of the primary system until the acknowledgement has be received from the secondary system. In addition, the primary volume (PVOL) in the primary storage system and the secondary volume (SVOL) in the secondary storage system are identically maintained, so that the SVOL can be used promptly used to replace the PVOL if the PVOL experiences failure. However, the primary and secondary storage systems cannot be placed too far apart, e.g., over 100 miles, under this mode. Otherwise, the storage system may not efficiently execute write requests from the host.
In asynchronous mode, a write request from a host to the primary storage system completes upon storing write data only to the primary system. The write data is then copied to the secondary storage system. That is, the data write to the primary storage system is an independent process from the data copy to the secondary storage system. Accordingly, the primary and secondary systems may be placed far apart from each other, e.g., 100 miles or greater. However, data may be lost if the primary system goes down since the PVOL and SVOL is not identically maintained.